1. Field of the Invention
The invention relates to the correction of an X-ray flat panel detector and a grid for elimination of scattered ray, used in a radiation imaging device for medical use.
2. Description of Related Art
In recent years, solid-state imaging devices, which are also called “flat panel detector (FPD),” have drawn a lot of attention. It is known that the operating methods thereof include a direct-type FPD and an indirect-type FPD. The direct-type FPD converts X-ray energy into an electric charge directly, and the electric charge is read by a readout element, e.g. a thin film transistor (TFT), as an electric signal. In contrast, the indirect-type FPD utilizes a scintillator to convert X-ray energy into a light, which is converted into an electric charge by a photoelectric conversion element, and then the electric charge is read by a readout element, e.g. a thin film transistor (TFT), as an electric signal. In either method, information of the imaged object, which is being collected on the surface of the detector, is read as the information that is spacially sampled according to the pitch of the readout element (known as a “detector pitch” hereinafter).
As shown in FIG. 16, when an imaged object 110 is irradiated by an X ray 111, a portion of the X ray 111 is absorbed by the imaged object 110, but the rest of the X ray 111 is not absorbed and reaches a detector 102 as a transmission X ray 112. In addition to the transmission X ray 112 that passes through the imaged object 110, a noise component, called a scattered ray 113, is also released from the imaged object 110. The scattered ray 113 reduces the SN ratio (Signal to Noise ratio) or the contrast of the imaging information of the imaged object 110, which is transmitted by the transmission X ray 112. In this situation, a grid 101 is usually used to eliminate the scattered ray as much as possible.
A structure of the grid 101 includes X-ray shielding materials 103 and intermediate materials 104, wherein the X-ray shielding materials 103 are stripe-like and respectively separated by an interval to accommodate the intermediate materials 104. Because the scattered ray 113 is absorbed by the X-ray shielding materials 103, the scattered ray 113 does not reach the detector 102. Thus, the SN ratio and the contrast of the image information can be enhanced. However, in a situation in which a secondary scattered ray 116 occurs in the intermediate materials 104, the secondary scattered ray 116 cannot be completely eliminated.
Generally speaking, the grid ratio or grid density is used as a value that represents the scattered-ray-elimination capability of the grid. The grid ratio and grid density are determined by the thickness C and the height A of the X-ray shielding materials and the thickness B of the intermediate materials, as shown in FIG. 17. Grid ratio is defined as r=A/B and grid density is defined as N=1/(B+C) [lp/cm]. The foregoing values are determined based on the types of the detectors and the usages thereof.
Grids are categorized into two types, for example, a movable grid and a fixed grid. In terms of the movable grid, the grid is simultaneously moved with the X-ray radiation in a direction of the grid stripes or in a direction perpendicular to the grid stripes, so as to avoid forming a fixed pattern of the grid in the image. In terms of the fixed grid, the imaging process is carried out with the grid fixed on the detector. When the fixed grid is used in the imaging process, the imaged object information that reaches the detector includes the grid stripes with the fixed pattern.
When using the movable grid, the grid stripes with the fixed pattern are not included. However, the X-ray shielding materials may cause a cutoff during the movement, which results in the deficiency of X ray and impairs the image quality. Also, a means is required to mechanically move the grid, and consequently, the device would be larger in size and the production costs thereof is increased. Furthermore, vibration caused by the movement and electric noise generated by the motor have great negative influence on the image.
On the other hand, when the fixed grid is used, it is necessary to perform a correction on the fixed pattern of the grid stripes. If the relative positions of the grid and the X-ray flat panel detector remain definite, the correction data that has been obtained in advance can be used to perform the subsequent correction.
However, when the grid is installed on or dismantled from the X-ray flat panel detector, misalignment may occur. As a result, the correction data that has been obtained in advance cannot be used for correction.
Accordingly, the following techniques have been disclosed: a marker is disposed on the grid, or an image signal is relied on to determine the position of the shadow of the X-ray shielding materials 103 to thereby infer the relative position relationship between the grid and the X-ray flat panel detector, and the correction is performed on the fixed pattern of the grid stripes based on the relative position relationship (see Patent References 1-3, for instance).    Patent Reference 1: JP 2001-134748    Patent Reference 2: U.S. Pat. No. 5,581,592    Patent Reference 3: U.S. Pat. No. 5,291,539
However, the shape of the shadow of the marker is subject to change with the position, etc. of the X-ray focal point. Due to the influence of noise, etc., the shadow may not have the same shape as the marker. Furthermore, even though an extension of the shadow of the marker can be determined by the processing threshold values of each pixel value, the value that should be set as the threshold value may vary according to imaging conditions and status of the imaged object. Therefore, it is difficult to determine the extension of the shadow, and as a result, the position of the marker may not be determined correctly.